The present invention generally relates to amplifying circuits and more particularly to a summing amplifier which adds and amplifies two input signals simultaneously. The usual method of amplifying signals from different sources is to combine them at low level stages of the amplifier. This method is generally useful and satisfactory for sine wave-like signals where either inter-modulation distortion is of little consequence or where such distortion can be minimized by careful circuit design. However, when it is necessary to minimize inter-modulation distortion and particularly where it is desirable to take advantage of the efficiency offered by switching on-off operation, the combining of signals at the amplifier input is at a disadvantage at high level operations. It is thus desirable to have a circuit design where the disadvantage of combining input signals at the amplifier input is overcome. Such a circuit is desirable for driving of a parametric acoustic source used for underwater communication wherein two frequencies must be simultaneously applied to a single source projector.